1. Field of the Invention
This invention relates to a calender roll for a supercalender used for smoothing or glazing the surface of a web by nipping the web between a metal roll and an elastic roll, and more particularly to an improvement in an elastic roll for a supercalender used at a working temperature of 60.degree. to 120.degree. C.
2. Description of the Prior Art
The supercalender is a calender consisting of a metal roll and an elastic roll nipping a web with a comparatively high linear pressure of 50 to 500 kg/cm for smoothing or glazing the surface of the web. Particularly when glazing the surface of a web, the linear pressure is increased up to 100 to 500 kg/cm and the working temperature is raised up to 60.degree. to 120.degree. C. Therefore, the elastic roll must have high pressure resistance and high heat resistance. In order to meet such requirements, there have been made various researches and investigations to improve the surface hardness or material of the elastic roll. Japanese Patent Laid Open No. 51(1976)-92606, Japanese Patent Laid Open No. 51(1976)-103404, and Japanese Patent Publication No. 52(1977)-17404 propose this kind of improvements.
The above proposed elastic rolls, however, merely suggest the surface hardness or material of the elastic rolls at the room temperature or non-working temperature. No proposal is made regarding the surface hardness of the elastic roll at the working temperature namely while the elastic roll is used for calendering a web. Particularly, the relationship between the surface hardness and the working temperature of the elastic roll it is not known at all.
For instance, in manufacturing a magnetic tape, it is known that it is very important to smooth the surface of the magnetic layer of the tape for enhancing the S/N ratio of the tape. In order to smooth the surface to a very high degree or to glaze the surface, it is required to raise both the linear pressure at the nip of the rolls and the temperature of the rolls. In Japanese Patent Publication No. 52(1977)-17404 and Japanese Patent Laid Open No. 51(1976)-92606 it is proposed to make the surface hardness of the elastic roll 70.degree. or more (Shore hardness D-scale), particularly 85.degree. or more.
It is mentioned therein that the desirable hardness can be achieved by polyurethane or polyamide. It is true that the above mentioned hardness can be achieved by these materials at a room temperature. However, the desirable hardness cannot be obtained by these materials at a working temperature or a raised temperature. Particularly, when the working temperature becomes 80.degree. C. or more, the desirable hardness of 85.degree. (Shore hardness) cannot be obtained.
With the elastic roll as mentioned above the surface hardness of which falls below 85.degree. at the working temperature it is impossible to perform the satisfactory smoothing and glazing of the tape. Further, it is considered impossible to perform the calendering for a long time at a high temperature. Therefore, the conventional supercalender is provided with a cooling means for cooling the elastic roll from inside or outside or cooling the web in order to prevent the elastic roll from being heated too much and enable the elastic roll to endure a long-term continuous working. However, while the supercalender is running for a long time, the surface temperature of the elastic roll gradually increases. Further, if the cooling efficiency of the elastic roll is made too high, the surface temperature of the magnetic tape or the like to be calendered is not raised sufficiently. Therefore, it is necessary to provide a heat-resistant elastic roll having a high surface hardness which has high resistant to a high temperature.